The invention generally relates to inkjet printers, and more particularly to a method of cleaning nozzles in an inkjet printhead.
Inkjet printers can be divided into two major categories, commonly referred to as continuous inkjet and drop-on-demand (DOD) inkjet.
In DOD inkjet printers, printing ink droplets are discharged from closely spaced nozzles in a printhead and onto a printing medium such as paper. Typically, the ink droplets are formed via thermal or piezoelectric activators, sometimes referred to as xe2x80x9cfiring devicesxe2x80x9d. With thermal activators, thin-film resistors or other type heater elements can be located in small firing chambers for the nozzles. When an electrical printing pulse heats a heater element, a vapor or gas bubble is formed between it and the nozzle inside the firing chamber. The bubble forces an ink droplet to be ejected from the nozzle. Then, when the heater element cools, the bubble collapses, and replenishment ink is drawn into the firing chamber due to the capillary attraction of the ink to the nozzle. With piezoelectric actuators, piezoelectric crystals or other piezoelectric elements can be located in the firing chambers. When an electrical printing pulse stimulates the piezoelectric element, it is mechanically actuated to cause an ink droplet to be expelled from the nozzle.
The ink delivery apparatus for the printhead in a DOD inkjet printer delivers very small quantities of the ink to the firing chambers in the printhead at a slight negative pressure or vacuum known as a xe2x80x9cback pressurexe2x80x9d. The slight negative pressure is desired because it prevents the ink from leaking, i.e. drooling, out of the nozzles by tending to draw the ink at the nozzles back into the firing chambers. Moreover, it forms a slightly concave ink meniscus at each nozzle which helps to keep the nozzle clean. Typically, as stated in prior art U.S. Pat. No. 5,650,811 issued Jul. 22, 1997, the slight negative pressure in the printhead may be approximately two to three inches of water below atmospheric pressure. The patent also states that the slight negative pressure can be created by positioning an ink reservoir for the printhead below the printhead. Alternatively, the slight negative pressure can be created by using a nonlinear spring to pull a compliant membrane outward at an opening in an ink reservoir above the printhead. This latter approach is described in detail in U.S. Pat. No. 4,509,062 issued Apr. 2, 1985.
A known problem with DOD inkjet printers is that dirt or dried ink can accumulate over time in the nozzles. Before this occurs, the nozzles should be cleaned such as by flushing the ink or a cleaning solvent under positive pressure outwardly through the nozzles. Otherwise, the dirt or dried ink can cause the ink droplets ejected from the nozzles to be misdirected with respect to the printing trajectories that the ink droplets should normally take. Such misdirection can cause the printed image to be of a lesser quality.
The cross-referenced application discloses a DOD inkjet printer in which an ink reservoir is positioned atop the printhead to provide ink delivery at a slight negative pressure to the printhead. A pressure regulator and ink replenishment mechanism maintains the slight negative pressure in the reservoir during ink delivery to the printhead, and in response to ink delivery provides comparable ink replenishment to the reservoir from an ink conduit projecting into the reservoir. The mechanism includes a compliant pressure regulator membrane that covers a wall opening in the reservoir and is connected via a rocker lever outside the reservoir to a compliant valve membrane that covers a different opening in the reservoir and normally caps the ink conduit to prevent ink replenishment to the reservoir. Ink delivery from the reservoir to the printhead causes the pressure regulator membrane to deform inwardly at the wall opening to decrease the holding volume of the reservoir, in turn to forward-pivot the rocker lever to deform the valve membrane outwardly at the other opening to uncap the ink conduit in order to initiate ink replenishment to the reservoir. When ink is replenished to the reservoir, the pressure regulator membrane returns outwardly to increase the holding volume of the reservoir, in turn to reverse-pivot the rocker lever to return the valve membrane inwardly to recap the ink conduit in order to terminate ink replenishment. The pressure regulator membrane maintains the slight negative pressure in the reservoir by being able to deform inwardly during ink delivery to the printhead and to return outwardly during ink replenishment to the reservoir.
A method of cleaning spaced nozzles in a printhead of a drop-on-demand inkjet printer in which a slight negative pressure is desired in an ink reservoir in order to prevent ink drool from the nozzles, comprising:
deforming a compliant pressure regulator membrane that covers an opening in an ink reservoir, inwardly at the opening, to decrease the ink holding volume of the reservoir;
deforming a compliant valve membrane that covers an opening in the ink reservoir and caps an ink conduit projecting into the reservoir, outwardly at the opening and away from the ink conduit, to uncap the ink conduit in order that the ink conduit can provide ink delivery at a positive pressure into the reservoir and out through the nozzles to clean the nozzles;
returning the compliant valve membrane inwardly towards the ink conduit to recap the ink conduit in order to terminate ink delivery into the reservoir; and
returning the compliant pressure regulator membrane outwardly to increase the ink holding volume of the reservoir in order to reduce ink pressure in the reservoir.
Also, the method can further comprise:
ejecting some ink from the nozzles by activating thermal or piezoelectric activators for the nozzles, when the compliant valve membrane has returned to recap the ink conduit, and not before the compliant pressure regulator membrane has returned outwardly to increase the ink holding volume of the reservoir,in order to ensure a slight negative pressure in the reservoir which prevents ink drool from the nozzles.